A membrane can be viewed as a semi-permeable barrier between two phases of liquid, gas, or liquid/gas. The membrane acts to prevent contact between the two phases and the semi-permeable nature of the membrane allows restricted diffusion of specific molecules such that a separation takes place. The movement of molecules across the barrier can be restricted in a very specific manner.
An immiscible liquid can serve as a membrane between two liquid, gas, or liquid/gas phases. Different diffusant molecules will have different solubilities and diffusion coefficients in a liquid, and therefore, yield selective permeabilities to achieve a separation (Noble and Way (1987) in Liquid Membranes: Theory and Applications, American Chemical Society 347:1-26). The use of a liquid membrane can result in a larger flux due to the higher diffusion coefficients of diffusant molecules in liquids than in solids. When an AC electric field is applied across a non-conducting liquid membrane, the diffusion flux across the membrane can increase sharply. This increased flux is not caused by an increased current, but by electrohydrodynamic (EHD) mixing (Hoburg and Malihi (1978) Phys. Fluids 21:2118-2119), which is a disordering of the fluid resulting in an alteration of mass transport across the fluid region. In other words, the increase results from a type of convection driven by electrical forces rather than phase changes or by heating of the fluid layer. Convection driven by electrical forces differs from other types of convection, such as free convection driven by gravity, or forced convection driven by mechanical forces (e.g., pressure) (Plonski et al. (1979) J. Membrane Sci. 5:371-374). Full disordering or turbulence of the liquid is obtained with a low AC field strength of 100 V/mm. Liquid crystals (LCs) or LC-like fluids are mesomorphic phase materials exhibiting characteristics intermediate between crystalline solids and true amorphous liquids. LCs are usually composed of strongly elongated molecules with a tendency toward ordering and alignment of the molecules characteristic of solid crystals but retaining relative motion and flow between the crystals. LCs or LC-like fluids retain their mesomorphic phase characteristics up to a transition temperature at which the fluid undergoes a transition to a normal liquid phase. LCs are classified in three categories according to their general symmetry, as nematic, cholesteric, or smectic. Below the transition temperature, the LC fluid exhibits dielectric anisotropy and electric conductivity anisotropy. These anisotropic physical properties can be modified with various physical or chemical agents locally or throughout with great facility, giving rise to numerous technological applications.
Hwakek and Carr (1987) Heat Transfer Eng. 9:36-69, and U.S. Pat. No. 4,515,206, issued May 7, 1985 to E. F. Carr, entitled: Active Regulation of Heat Transfer, used electroconvection to regulate the passage of heat flux through a Nematic Liquid Crystal (NLC), demonstrating a field induced enhancement of effective thermal conduction by a factor of 25. The NLCs are liquids characterized by long range ordering of the long axes of their rod shaped molecules. NLCs, by virtue of their fluidity and intrinsic anisotropy, exhibit dramatic EHD effects at low applied electric fields (de Gennes (1974) Liquid Crystal, Oxford Press, London; Chandrasekhar (1977) Liquid Crystals, Cambridge University Press, Cambridge: Orsay Group on Liquid Crystals (1971) Mol. Cryst. Liq, Cryst. 12:251). A few hundred volts/mm can generate fully developed turbulent convection in appropriately designated NLCs.
N-(4-methoxybensylidine)-4-butylaniline (NBBA) is a typical liquid crystal having negative dielectric anisotropy. Zn the absence of an applied field, it is at rest and gas transport across the membrane is limited by molecular diffusion in the liquid crystal. When an electric field is applied, charge accumulates at the walls (defects), which are perpendicular to the electrodes. Forces due to the interaction of the electric field with the space charge at the wall tend to shear the sample. When the direction of the electric field is alternating, the walls are always charged in the alternating direction of the director. An AC field of approximately 100 V/mm produces fully turbulent flow. This chaotic flow disorders the NLC, generating disclinations in the molecular orientation field which strongly scatter light, producing the so-called "dynamic scattering" LC electro-optic effect (Berne and Pecora (1976) Dynamic Light Scattering, Wiley, New York). The disclination lines generated by EHD flow can be observed optically. The lines form parallel to the flow velocity, indicating the flow of the NLC perpendicular to the electrodes and back and forth between them. As applied, EHD flow mixes the LC layer, convecting dissolved species across the LC layer and forming an eddy diffusion process, thereby enhancing its apparent permeability. There have been several previous studies of electric field effects on gas permeation through liquid layers. Kajiyama and co-workers (Kajiyama et al. (1982) J. Membrane Sci. 11: 39-53; Washizu et al. (1984) Polym. J. 16:307-316; Kajiyama et al. (1985) J. Memb. Sci. 24:73-81; Shinkai et al. (1986) J. Chem. Soc., Chem. Commun., p. 933; Kajiyama et al. (1988) J. Membrane Sci. 36:243-255; Kajiyama (1988) J. Macromol., Sci. Chem. A25(5-7):583-600; Qiao and Wang (1987) Membrane Sci. & Tech. (Ch.) 7:1-7) have demonstrated permeation control in NLCs confined in polymer composite structures, using applied electric fields to orient molecules of the NLC and exploiting the anisotropy of the diffusion coefficients. The use of EHD stirring to facilitate mass transfer across a fluid membrane has been demonstrated by Plonski et al. (1979) Jo Membrane Sci. 5:371). In those experiments, the ion flux through a nonconducting (octanol) film separating aqueous ionic solutions was controlled by a factor of 10 by an applied electrical field. However, the large field required to alter ion flux made the films unstable.
The modification of LC structure has been used in the controlled release of drugs [U.S. Pat. No. 4,513,034 issued Apr. 23, 1985, to R. V. Sparer, entitled: Variable Permeability Liquid Crystalline Membrane; U.S. Pat. No. 4,968,539, issued Nov. 6, 1990, to Aoyagi et al., entitled: Liquid Crystal Membranes]. Sparer describes a LC layer contained in a porous structure which provides access to the LC layer to different molecules. The flow of solutes or permeants through the membrane is regulated through application of electric, thermal, or magnetic fields, which serve to alter the phase of the LC. For example, an electric field with a strength of 300-500 volts per centimeter causes the liquid crystal layer to change from the cholesteric to the nematic phase at room temperature. The membrane of Aoyagi et al. is comprised of a hydrophobic polymer membrane upon which is immobilized a liquid crystal-forming compound which has a transition temperature between 25.degree.-45.degree. C. A heating member applies an electric field to the LC layer, heating the LC layer above the gel/LC transition temperature, resulting in diffusion of a drug out of a drug reservoir layer.
In one configuration of a liquid membrane, a liquid is impregnated in the pores of a porous solid for mechanical support. This form is commonly known as an immobilized liquid membrane (ILM) (Noble and Way (1987) supra). The ILM has been recognized as an effective technology to simplify the process of creating an interface between two phases and recovering the products of separation. Selective transport across the ILM can be facilitated by carriers. However, there are two primary problems associated with the use of ILMs. Solvent loss can occur through evaporation, dissolution, or large pressure differences forcing solvent out of the pore support structure. Further, carrier loss can occur due to irreversible side reactions or solvent condensation on one side of the membrane. Pressure differences can force the liquid to flow through the pore structure and leach out the carrier (Noble et al. (1989) Chem. Eng. Prog. 85:58-70). These problems decrease the ILM's lifetime and have limited its successful commercialization.
As stated above, the use of a liquid phase can enhance the solute flux due to the higher coefficients in liquids than in solids. Further enhancement can be accomplished by using a nonvolatile carrier in the liquid (King (1987) Chapter 15 in Handbook of Separation Process Technology (R. W. Rousseau, ed.), Wiley-Interscience Publishing Co. This carrier molecule can selectively and reversibly react with the solute. This reversible reaction provides a means of enhancing the solute flux and improving the selectivity at the same time. By combining the advantages of high diffusion coefficients in liquids with the added carrying capacity of the carrier, larger fluxes can be obtained in liquid membranes than in polymer membranes. The selective nature of the carrier provides much better separations than those obtainable solely on relative solubility and diffusion.
Electrochemical processes have been used for chemical separations (Newman (1973) Electrochemical Systems. Prentice-Hall, Englewood Cliffs, N.J.). The most general applications are electroplating of metals in the processing of ores and the formation of metal coatings. In cases where a redox process altered the thermodynamics of a reversible complexation reaction, electrochemical cycles have been devised that result in separation for different species. Koval et al. (1988) Separat. Sci. Technol. 23:1389-1399, devised a mechanism that combines electrical energy and reversible complexation for the removal of sulfur and nitrogen compounds from a feed organic phase and subsequently concentrates them in a receiving organic phase using an equilibrium stage process. The core of their separation process is the reversible reaction between complexing agents (or carriers) and the sulfur and nitrogen compounds. The process which uses electrochemistry to modulate the complexation reaction is termed Electrochemically Modulated Complexation (EMC).
In an EMC process, a complexing agent, dissolved in the contacting (aqueous) phase, is electrolyzed to its high solute affinity redox state. The solute is extracted from a feed phase by partitioning into the contacting phase via reaction with the complexing agent. The complexing agent is then electrolyzed to its low solute affinity redox state and the solute partitions into the receiving phase upon contact with the aqueous phase. The contacting phase is then recycled.
In the EMC process, the complexing agent must meet four requirements: (1) it must be soluble only in the contacting (aqueous) phase in order to prevent any loss; (2) it must have a solute binding site and it must undergo a chemically reversible redox cycle in the presence and absence of the solute; (3) a considerable difference must exist in the affinity of the solute for the complexing agent in its two oxidation states; and (4) the kinetics of the solute-complexing agent reaction should be sufficiently rapid with respect to interfacial mass transfer. Complexing agents which meet these requirements include metal chelates which reversibly bind gases like CO.sub.2, CO, or H.sub.2 S. These metal chelates contain iron, copper, or cobalt (e.g., primary transition metals). Suitable complexing agents include iron or copper porphyrins which are soluble in an organic phase and contain a metal center.